Temperature compensated current sources have been utilized for numerous applications. One primary application is to provide compensation for a current controlled oscillator. In these applications, a current source is utilized to supply current to one side of a capacitor. The voltage on the capacitor is then input to one side of a comparator, the output thereof providing a periodic waveform operating at a predetermined frequency. When the voltage on the capacitor rises above a predetermined threshold, the output state of the comparator switches, this switching operation discharging the capacitor, which is then charged up to the switching voltage again. The frequency of switching on the oscillator output is a function of the size of the capacitor and the current supplied thereto. This current is a constant current. However, temperature variations in the current that drives the capacitor will cause temperature variations in the frequency of the output waveform. Although there are some inherent temperature variations associated with the capacitor and the circuitry associated with the oscillator, the primary source of temperature variation in the output waveform is the current supplied to the capacitor.
A number of current sources have been utilized to provide for temperature compensated current. For example, in U.S. Pat. No. 4,978,930, a Temperature Proportional to Absolute Temperature (PTAT) is generated in addition to a temperature stable current. The PTAT current is then subtracted from the temperature stable current to provide an offset current. This offset current is inversely proportional to temperature. This offset current is utilized to offset a control signal voltage to form a tuning signal for an oscillator. However, this solution requires the generation of a temperature stable current in addition to the generation of the PTAT current. The temperature compensation is therefore dependent upon how accurately the temperature stable current is generated.